The use of dichlorodifluoromethane (CFC12) for refrigerators and automotive air conditioners has recently been legally regulated to protect the ozone layer, and will be totally banned. Thus, hydrofluorocarbons which do not destroy the ozone layer, such as 1,1,1,2-tetrafluoroethane (HFC134a), have been developed as substitutes for CFC12.
However, since the polarity of hydrofluorocarbons is higher than that of CFC12, the use of conventional refrigeration lubricating oils, such as naphthenic mineral oil, poly-.alpha.-olefin or alkylbenzene, causes two-layer separation of the working fluid at low temperatures. This is due to the poor compatibility between the conventional lubricating oils and hydrofluorocarbons. Two-layer separation hampers oil return, which in turn interferes with heat transfer due to the deposition of a thick oil film around the condenser and evaporator which serve as heat exchangers. It can also cause important failures such as poor lubrication, and foaming upon starting operation. Therefore, the conventional refrigeration oils cannot be used in the presence of these new refrigerants.
As for lubricity, CFC12 generates hydrogen chloride upon partial decomposition. The which hydrogen chloride thus formed reacts on the friction surface to form a coating of chloride which provides a lubricity improving effect. On the other hand, hydrofluorocarbons are not expected to have such an effect because they contain no chlorine atom; therefore, the refrigeration oils used in combination with hydrofluorocarbons are required to have better lubricity than that of the conventional refrigeration oils.
In addition, the refrigeration oils used in combination with hydrofluorocarbons need to have good thermal stability in the presence of hydrofluorocarbons.
Moreover, since organic substances are present in the compression refrigerating machines for electric refrigerators as materials for motor components such as insulators and enameled wires, the working fluid comprising a hydrofluorocarbon and a refrigeration oil must not adversely affect these organic materials and must have a good insulating property.
Polyether compounds which can be used as refrigeration oils in combination with hydrofluorocarbons such as HFC134a are disclosed in U.S. Pat. No. 4,755,316, Japanese Patent Laid-Open Nos. 198694/1989, 256594/1989, 259093/1989, 259094/1989, 259095/1989, 84491/1990, 102296/1990, 129294/1990, 132176/1990, 132177/1990, 132178/1990, 132179/1990, 173195/1990, 180986/1990, 180987/1990 and 182780/1990 and other publications.
Since polyether compounds have a polarity higher than that of naphthenic mineral oils, their compatibility with HFC134a at low temperatures is really good. However, polyether compounds cannot be safely used as refrigeration oils, since they pose a problem of two-layer separation upon rise in temperature, as stated in U.S. Pat. No. 4,755,316.
Polyether compounds involve other problems as well. Among them is their poor insulating property. This is a serious a very important problem which makes it impossible to use polyether compounds for refrigerating machines used in electric refrigerators. Another problem is their high hygroscopicity. The moisture in the polyether compound degrades the thermal stability in the presence of HFC134a and causes organic materials such as PET films to be hydrolyzed.
Moreover, polyether compounds are not satisfactory as to their lubricity; systems of a hydrofluorocarbon refrigerant such as HFC134a and a polyether compound are inferior to conventional systems of CFC12 and a naphthenic mineral oil.
On the other hand, there have been proposed some methods using an ester in combination with fluoromethane in the field of refrigerating machines, including those disclosed in Japanese Patent Laid-Open Nos. 131548/1981, 133241/1981, 181895/1986 and 592/1987 and other publications. Also, Japanese Patent Laid-Open Nos. 125494/1981, 125495/1981 and 62596/1986 describe the use of an ester in mixture with another lubricating oil. Examples of the use of an ester in the presence of an additive are described in Japanese Patent Laid-Open Nos. 155093/1980, 36570/1981, 125494/1981, 15592/1983, 103594/1983, 171799/1986 and 292895/1987.
However, these methods are all based on a system used in combination with a dichlorodifluoromethane (CFC12) or monochlorodifluoromethane (HCF22) refrigerant, and none of them makes mention of a hydrofluorocarbon refrigerant. In addition, their object is to improve the thermal stability in the presence of CFC12 or HCFC22.
Japanese Patent Laid-Open Nos. 143609/1978 and 164393/1984 describe the compatibility with fluoromethane as well as the improvement in the thermal stability in the presence of fluoromethane, but their object is to avoid excess dissolution in CFC12 in the former case and in HCFC22 in the latter case, and no description is given on the improvement in the compatibility with hydrofluorocarbon refrigerants.
Methods using a hydrofluorocarbon such as HFC134a and an ester in combination are described in U.S. Pat. No. 4,851,144 and Japanese Patent Laid-Open No. 158693/1990. However, both of these publications both describe the use of a polyether compound partially mixed with an ester in combination with HFC134a; and neither of them makes mention of the use of an ester alone nor is there a description of the object of adding an ester.
As stated above, as far as the prior art is concerned, none of the existing compositions comprising a hydrofluorocarbon and oil for refrigerating a machine used in electric refrigerators etc., meet the requirements for compatibility, lubricity, thermal stability, insulating property and other properties.